1. Field of the Invention
The present invention relates in general to hydraulically expanded flow channels, and, in particular, to a reinforced hydraulically expanded flow channel which prevents the width of the hydraulically expanded flow channel from changing and as a result reduces the strain rate in the high strain area, and increases the life of the unit.
2. Description of the Related Art
Hydraulic expansion manufacturing techniques are known for creating flow channels. U.S. Pat. No. 4,295,255 issued to Weber describes a method of manufacturing a cooling jacket assembly for a control rod drive mechanism. This technology has been further applied to creating a flow channel as shown in FIG. 1. This type of flow channel has been used as a coiled-tube for a boiler. The flow channel finds utility in many applications, for example, in a stored chemical energy propulsion system (SCEPS) as described in U.S. patent application Ser. No. 07/666,276 filed Mar. 7, 1991, and now U.S. Pat. No. 5,138,765 issued Aug. 18, 1992, hereby incorporated by reference.
To fabricate a flow channel (inner or outer helical coil), one cylinder (12) is placed inside another cylinder (14) and an electron beam welder (not shown) spirally welds in a helical weld path (16) the two cylinders (12, 14) together. After welding, hydraulic pressure is applied between the welds (16) of the two cylinders (12, 14). As the hydraulic pressure increases, the cylinders (12, 14) deform between the helical weld path (16) creating a long, continuous flow channel (18) as shown in FIG. 1.
It is also known to roll two metal sheets into a cylindrical shape to fabricate the cylinders. The cylinders are assembled with a tight mechanical fit radially so that there is no gap, with an interference type fit. As mentioned earlier, the inner cylinder is joined to the outer cylinder by welding through the wall along a helical path, and the end welds are made to close the helical path. More than one helix may be welded to form multiple paths. Next, one of the cylinders is penetrated to the interface and a pressurization line is attached. By pressurizing the interface, the cylinders are expanded apart between the welds to form the flow channel. This may be done hot with gas, or cold with a liquid.
During the expansion, the initial straight-line interface between the cylinders expands into an eye-shape and becomes closer to round as the expansion continues, note FIGS. 2a-f. These figures show a finite element model at various stages of the expansion process.
The high strain area is next to the weld (16) in the tight radius bend area. As is apparent from FIGS. 2b-f, the strain increases as the expansion process continues. Experimental results for a given geometry, material and test temperature, indicate that failure occurs at approximately the same expansion or strain level independently of the pressure/time cycle.
In manufacture, the part is expanded to a strain level less than the failure strain. The rupture life of the part at a given temperature and pressure depends on the difference between the rupture strain and the expansion strain as manufactured. Rupture life is correlated as follows: EQU LIFE=A.times.(1-LRUPTURE/LEXPANDED).sup.n
where:
LIFE is the rupture life PA1 LRUPTURE is the cylinder length at rupture, and PA1 LEXPANDED is the cylinder length after expansion.
The strain correlates with length or channel width. As the flow channel expands, the width of the channel and the length of the cylinder decrease as the strain increases. These values were A=781,753 and n=2.3674 for a studied case.
In current applications, the expanded coil (10) is used without axial constraint. In service, the flow channel (18) continues to expand based on the operating temperature and pressure until failure occurs.
Thus, it is desirable to prevent the length of the coil from changing and to reduce strain rate in the high strain area next to the weld.